1. Field of the Invention
This invention relates to the reuse (i.e. not recycling) of slag, a waste by-product of the metals refining industry, through specialized treatment to create high-value by-products. In particular, it relates to controlled casting and forming of slag to create structural and decorative construction materials, such as tiles, bricks, blocks, etc. The structural and decorative construction materials of the present invention can be used for interior and exterior of buildings; for use as low-impact, high-abrasion wear linings (e.g. in chutes, etc.); non-skid applications (e.g. flooring and tiles); and for use in almost all current applications of pre-cast concrete.
Traditionally, slags of the steel industry have been slow-cooled or water-quenched and reused as construction aggregate. This traditional method of cooling results in coarse crystals and other inhomogeneities in the slag. While this may be quite satisfactory for road aggregate, it is not typically desirable for uses in buildings where high strength and wear resistance are required.
The present invention reduces the inhomogeneities in the final slag product by regulating the temperatures and lengths of the `fining` and `forming` cycles, as well as by controlling the casting temperature and slag additives. The resulting product is strong, stable and structural, formed into various desired shapes, suitable for building and construction materials and other applications requiring high-strength, low-abrasion or non-skidding.
2. Description of the Prior Art
As noted above slags of the metal refining industry have traditionally been slow-cooled or water-quenched and reused as construction aggregate. Some efforts have been made to develop alternative uses for the slag as structural and decorative construction materials. None of these prior efforts has to date been commercially successful.
Canadian Patent No. 326838 describes a process of directly casting molten slag into moulds for shapes suitable for construction. In this process molten slag is tapped into a cylindrical vessel where it is stirred to liberate entrapped gases and cooled to lower its viscosity. These evolving gases create gas pockets within the slag, and lower viscosity is sufficient to hold the bubbles in the molten viscous or plastic state so that a solidified foam or cellular structure eventually results. This foamed slag is tapped and cast into moulds in shapes of building blocks or bricks.
The resulting solidified slag product is not likely to consistently form a strong structural building material. Casting slag directly from molten temperatures to ambient temperatures results in thermal stresses in the final product. This final product either spontaneously shatters upon cooling, or at best is very weak structurally. This rapidly cooled material can vitrify (i.e. become glassy or amorphous) rather than crystallize. In addition, this method claims to be able to create a dense structure (i.e. with little or no porosity) by circumventing any gas evolution from the slag during cooling. However, the creation of a dense structure without voids requires complete entrapped gas evolution; without extensive gas evolution, any untreated slag produces gas bubbles in the final product whether desired or not. Finally, this method of treatment does not include any compositional modifications to the slag (i.e. additions intended to change the overall chemistry).
In another process developed in the UK and known as Slagceram, decorative and structural construction materials, especially tiles, are made by a process of glass-making and controlled devitrification. Simply, a glass product is formed from thermally and compositionally treated blast furnace slag and cooled to ambient temperature. Then this product is again thermally treated to devitrify the slag; that is, to cause the glass structure to become crystalline. In this process molten slag is tapped from a blast furnace or transfer vessel into a slag treatment vessel. This treatment vessel is heated by some means, such as natural gas or induction. To form a molten glass, sand is added into the molten slag in a ratio of 30 parts sand to 100 parts slag. A nucleating agent is also added to induce uniform crystal growth. The entire mixture is held for a number of hours at a constant temperature (1430.degree. C. for five (5) hours), the `fining` period, to allow volatiles to escape (e.g. entrapped gases) and ensure complete dissolution of the additives. When the fining time has elapsed, the slag glass is poured and shaped below its glass transition temperature into the final product form (e.g. tile) using glass-making machinery. The slag glass product is thermally cycled for a number of hours to ambient temperature, and then moved to a secondary heat-treatment furnace.
In this furnace, the glass product is heated just above its glass transition temperature to the nucleation temperature, where the added nucleating agents will create sites for crystalline grain growth. After an appropriate number of nucleation sites have been formed, the temperature is increased to the grain growth temperature where small grains are grown from these sites. The resulting slag glass ceramic product is cooled to ambient temperature, ready for use, or enameled if desired.
The emphasis of the Slagceram process is to create a glass ceramic; that is, to first make a formed glass product with a number of nucleating agents, and then thermally treat the product to allow nucleation and crystalline grain growth.
U.S. Pat. No. 5,720,835 relates to the manufacture of a multi-layer tile, using slag as one of the potential components of the lower, strength-giving layer. The multi-layer tile consists of at least two layers of different material sintered together, where the top layer is made of sintered glass or other more decorative, less strong material, and the underlying layer or layers are composed of stronger materials (sand, slag granulate, crushed glass granulate) sintered together with a bonding agent that gives the tile its overall strength.